The present invention relates to apparatus for manipulating parisons in blow molding machines, and more particularly to apparatus for manipulating parisons in machines for forming bottles or analogous hollow articles of plastic material by injection and subsequent expansion. Still more particularly, the invention relates to improvements in machines wherein parisons are formed in a parison mold, wherein the parisons are thereupon transported (preferably by a blowing mandrel) from the parison mold to an intermediate station at which the material of the parisons is conditioned prior to expansion, and wherein the parisons are finally introduced into a blow mold for conversion into bottles or the like.
It is known to form bottles or analogous hollow shaped articles in a machine wherein a metered quantity of plasticized material is injected into a parison mold at a first station while the parison mold confines a portion of a blowing tube or mandrel so that the resulting parison surrounds and adheres to the mandrel. An advantage of such mode of making parisons is that each parison has a closed end prior to introduction into the blow mold and also that the parisons can be produced with a high degree of reproducibility. The conversion of such parisons into bottles or the like takes place at a second station which is spaced apart from the first station. As a rule, the parisons are transferred to the second station by the blowing mandrel which admits a gaseous blowing medium into the parison therearound as soon as the blow mold at the second station is closed.
It is further known to condition the material of the parisons prior to introduction into the blow mold. Such conditioning normally involves cooling (tempering) of the parison so as to change the condition of the material of the parison from a thermoplastic to a thermoelastic state. The conditioning or tempering can take place in the parison mold, i.e., the parison is cooled in response to contact of its external surface with the surface surrounding the cavity of the parison mold and in response to simultaneous contact of its internal surface with the exterior of the blowing mandrel. The just described mode of conditioning parisons is quite satisfactory; however, the procedure is ill adapted to high production rates because the output of the machine is relatively low since the making of the next-following parison can begin only upon completion of conditioning of the preceding parison.
Attempts to increase the output of the just described blow molding machines wherein shaped articles are produced by injection and subsequent expansion include the provision of an intermediate or conditioning station at which successive parisons dwell during transport from the injection molding to the blowing station. As a rule, the conditioning operation involves a cooling of the parisons; however, and depending on the shape of the final product, it is often necessary or desirable to heat selected portions of the parison while the remaining portion or portions of the same parison undergo a cooling treatment. Reference may be had to German Offenlegungsschrift No. 2,400,951 which discloses a tempering mold disposed between the parison forming and blowing stations and having means for heating and/or cooling a parison therein. The mold is similar to conventional molds, i.e., it has two sections which can be moved between open and closed positions and, when closed, confine a parison which is closed at one end and surrounds a blowing mandrel. The latter serves as a means for transferring the parison from the parison mold into the mold at the conditioning station, for thereupon transferring the parison from the conditioning station into the blow mold, and for admitting a gaseous blowing medium into the conditioned parison at the blowing station. The tempering effect of the mold at the conditioning station is more pronounced if the mold is formed with a cavity bounded by a surface which closely follows and contacts the entire external surface of the parison on the blowing mandrel. This presents a number of serious problems which are attributable primarily to sensitivity of the parison prior to conditioning. The material of the parison is highly sensitive to mechanical stresses because it is still in a thermoplastic state and is likely to be deformed in response to contact with the mold at the conditioning station. Moreover, the configuration of the external surface of the parison on the mandrel which is moved from the parison-forming station to the conditioning station depends on certain other factors (i.e., not only on pronounced plasticity of the freshly injected material), especially on elastic memory of such material. Therefore, prior art proposals include the utilization of tempering molds having cavities which can receive a parison with a certain amount of clearance in order to avoid undesirable deformation of parisons during conditioning, particularly the development of flashes or webs in regions where the sections of the tempering mold abut against each other. This, coupled with automatic shrinkage of parisons during cooling, results in insufficient contact between the external surface of the parison and the internal surface of the closed tempering mold so that the conditioning of parisons in such molds takes up a relatively long interval of time. Furthermore, the length of intervals for adequate conditioning varies from parison to parison because the extent of surface-to-surface contact between the tempering mold and successive parisons varies within a wide range. Consequently, the conditioning action of the tempering mold upon successive parisons is not uniform, not only as considered in the longitudinal but also as considered in the circumferential direction of the parisons. This affects the quality of the ultimate products.